Blue is a dark blue/black-colored wolfdog. At the start of the series, Blue is shown as Quent Yaiden's pet and aide in hunting down wolves, believing, just like her master, that wolves were to blame for the destruction of her home and family in Kyrios. She lived her life as a pet dog, unaware of her heritage, and helps Quent hunt down every last wolf. However, upon meeting Cheza, her wolf blood is awakened and she is unable to continue hunting down members of her own kind, and chooses to join the pack in the search for Paradise instead.

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Like Toboe, Blue is extremely fond of and attached to humans, having lived with them her whole life. She was extremely devoted and obedient to Quent and followed his commands without hesitation, even if it may have meant her own demise. She broke free of following Quent's orders once her inner wolf had awoken when she had contact with Cheza, unwilling to kill her own kind any further and afraid of how Quent would cope with her being around.

She is a fairly calm, level-headed individual. Although she seems laid-back, she will become fiercely protective and angered if anyone endangers anyone close to her. In the pack, she grows closest to Hige, and eventually, she ends up falling in love with him. Despite the possibility that Paradise won't accept her because she is a half-breed, she promises to follow Hige, no matter what happens.

It is unknown about Blue's origins of where she came from and what her birth name was, but it is known that as a pup she was found by Ruus Yaiden who took her home and was raised in the small town of Kyrios with his family. At first his father Quent was reluctant about keeping her but eventually gave in after Ruus's begging. Where she then went to live on happy with the family and was dubbed the name "Blue" due to the colour of her own fur. Unknown to the family and Blue herself, Blue was actually half wolf.
One unexpected day Kyrios was set on fire, fortunately, Blue was saved by Quent who barely managed to get out alive, Ruus and his Mother weren't so lucky and died. Before leaving off, Quent and Blue went to take one final look at their destroyed town and saw wolves in the horizon feasting on the burned corpses in the fire which lead the two to believing that the wolves were responsible for the deaths of all Kyrios. Together, they vowed to kill every last wolf as vengeance for their family's death.

Blue and Quent are traveling in the snowy blizzards where they come across an Arctic Wolf and attempt to kill it, Quent only managed to fire a grazing shot at the Wolf, they then follow the Wolf's footprints to Freeze City.

Upon arriving at Freeze City Quent goes into a bar to get a drink and have a conversation with the bartender and leaves Blue outside to rest. While asleep, Blue awakens to the sound of people breaking stuff and see's a line of people walking past the alley she's resting in, one of them stops to look at Blue and she gets the scent of this person being a Gray Wolf so she barks to get Quent's attention. Quent goes out to the look but finds the assumed wolf gone, however he finds wolf footprints on the dirt ground, confirming that there was a wolf present.

After a day spent in the city, Quent and Blue find the Arctic Wolf from the previous day again and attempt to kill it once more, Blue distracts the wolf whilst Quent shoots it in the stomach. Due to the hassle, Quent is brought in for questioning with unsatisfactory that the Wolf is apparently confirmed dead and people disbelieving him that it was in fact, Wolf as there has been no sighting since 200 years.

As Quent and Blue leave the building, they look back when Blue senses Cheza's blood reacting to the Wolf. The two go back to a hotel room that Quent rented.

The next day when Quent buys a drink and sits down to take a break, he tells Blue that they cannot make the same mistake as before on letting the Wolf escape and so they have to be more wary and be sure to not let them go.

Whilst walking in an alleyway, Blue spots the same person from the previous day who she identified as a Gray Wolf as well as another person who she also identifies to be a Red Wolf with him. A drunk Quent eventually see's through his gaze that the boy is in fact a Wolf, this refuels his full hatred for Wolves knowing that they take on the form of Human's, Quent goes to shoot the boy but is stopped by the other Humanized Wolf who claws his hand, Blue tries to bite him but misses. The two Boy's then jump an amazing height onto the buildings next to them, escaping.

Later on the afternoon of that day, Police Detective Hubb Lebowski (the officer who interrogated him earlier when brought in for questioning) visits Quent as he is taking his information on Wolves more seriously and to get some advice on the situation, Quent informs him of his sighting to more Wolves in Humanized forms, however due to his drunken status, Hubb decides to leave with getting no effort on what the Boy's looked like in order to keep an eye out and getting no advice from him. Before leaving Hubb informs Quent that the Arctic Wolf he and Blue got escaped and is in fact alive much to Quent's satisfaction, Quent tells Hubb to just keep an eye as anyone anywhere could be a Wolf seeing as they are taking the forms of Human's and also quotes the book of the red moon to Hubb about Wolves being the creation of Human's.

While Blue and Quent are on another walk around the streets, they overhear a conversation with a little girl and her father about seeing a "Dog" appearing as a Boy and wants to look for him, Quent and Blue follow behind. As they are following and when the girl screams that she see's it, Quent draws his weapon out to get the possible Wolf, however, Blue senses the Gray Wolf's scent again and splits up from Quent to go after and find it whilst he takes care of the sighted possible Wolf.

Blue finds the Gray Wolf running from the Police and stands in his way, the Gray Wolf makes a sarcastic compliment towards her saying "you're pretty good" and tries to run away when Police get closer, however Blue manages to bite his arm and falls head first into a stocked up pile of cardboard boxes full of potatoes. The Wolf gets away and Blue is injured. Quent finds her as well as noticing Gray fur on her and as well as a blood trail.

Quent leaves Blue behind at the hotel to rest from her injuries while he goes out to find the Wolves and inform Hubb more about the situation.

After recovering, Quent decides that the two should move on as the wolves they were after have left the city, so he takes her to rest outside the bar the two went to they first arrived at freeze city whilst he goes to get a drink. Whilst sleeping she wakes to Hubb standing outside the bar looking for Quent which makes it clear to him that Quent is inside. Hubb encounters Quent about the story in the "Book of the Moon", wondering if it is really a fairy tale, Quent gives his belief on the story being reality but let's Hubb decide on it himself. Before Quent leaves off with Blue, Hubb informs Quent that his ex-wife Cher Degré has gone missing and she is the one who left the Book of the Moon copy with him, Quent gives his final thought on the world ending to Hubb as the moon will turn red and burn, he then tells Hubb to do what matters to him soon before anything happens, such as finding Cher if he really cares about her whilst Quent himself along with Blue will go hunt down the Wolves before it ends.

Quent and Blue hitch a bus ride to a town on the cliffs, upon arriving Quent goes into a bar to get a drink, there he finds Cher along with Lord Orkham's troops. Blue stays outside to rest, she awakens to the scent of all three Wolves (along with one more being a Mexican Wolf) from earlier along with the flower maiden Cheza, before barking to get Quent's attention she notices the flower maiden and stays silent, but upon seeing the same Arctic Wolf from earlier her anger returns. Cheza however, comes over to Blue and reveals to her that she is a Wolfdog, much to Blue's shock, as Cheza and Wolf run away, Blue attempts to go after them but is held back by her leash, Lord Orkham's troops and Cher who come out of the bar notice a quick glance of the Wolf and Cheza running and go after them.

In the morning, a tired Quent looks at a picture of his family, reminding Blue about how Ruus found her and brought her home, and mentioning how Ruus inherited his stubbornness. Quent points out how big Blue has grown and pets her before drifting off to sleep.

After Quent falls asleep, Blue's leash falls from his hand. Blue hears yells and goes to investigate. She finds Lord Orkham's troops trying to capture a young boy. One of the soldiers knocks the boy into the wall, and he falls to the ground unconscious and bleeding. Blue growls at the soldiers, one of them tries shooting her but she knocks the gun out of his hands. Both soldiers run away.

Blue goes to check on the boy and licks his head. Quent comes to find her, and when he sees her standing over the boy he has a flashback to Ruus's death and the destruction of his town. Quent almost shoots Blue, and tells her to back away. She moves away, and Quent checks the boy's injury. He takes the boy back home and treats his injury.

While Quent is inside the boy's home tending to his injuries, Blue waits outside tied up. She wakes up when the two soldiers from before walk by. The soldier that she knocked down earlier approaches, while the other one continue with the mission. The soldier beats Blue in the head with the butt of his gun as revenge for scaring him. Blue continues to take the hits until finally she breaks free of her leash. The man shoots her, breaking her collar, and she kills him. Quent later finds the bloody corpse of the soldier and Blue's broken collar, with Blue nowhere in sight.

Quent later finds and chases after the wolves, while chasing them he finds Blue badly injured and bleeding. Blue collapses and he kneels at her side. When he hears the wolves howling he leaves her to continue the chase. Blue tries to stop him by tugging on his coat, but he promises to come back and leaves. After failing to kill the wolves he returns to where he left Blue, but she has already left.

Blue is afraid that Quent will not want to keep her if he knows she is part wolf, so she begins a journey of her own. She doesn't have a destination, but during her time alone she discovers how to use her human form.

Blue eventually encounters a group of human's in a black market looking for workers, after informing her that they will give her work she finds out that they likely only want her organs. Blue escapes and is chased by the men. She runs into a building where she finds Kiba, Hige, Toboe, and Tsume resting. Together, the wolves fight and scare away the humans. Hige invites her to join them on their search for Paradise, but she declines the offer.

After Lord Darcia III turns into a wolf, Blue attempts to kill him in vengeance for killing Quent Yaiden, but Darcia gets the upper hand and crushes Blue onto her back, breaking her spine. Hige attempts to help Blue but is knocked away by Darcia, who then bites Blue's throat, which eventually kills her.

Hige is with Blue in her final moments, she dies quickly from her injuries with him gently rubbing her head and bleeding out beside her.